Step 1

For the bechamel: Melt the butter in a large saucepan over medium heat. Whisk in the flour to form a smooth paste. Cook, whisking constantly, for 3 minutes. Slowly whisk in the milk until smooth. Add the salt and bay leaf and bring the mixture to a simmer, whisking occasionally. Reduce the heat to low and simmer, stirring occasionally to prevent burning, for 20 minutes. Remove and discard the bay leaf.

Step 2

For the lasagna: Meanwhile, add the oil and bacon to a large skillet over medium heat and cook, stirring often, until the bacon is crispy and golden brown and the fat is rendered, about 15 minutes. Using a slotted spoon, remove the bacon to a paper towel lined plate. Drain off all but 3 tablespoons of the fat from the skillet. Add the mushrooms and 1 teaspoon salt to the skillet and increase the heat to medium-high. Cook, stirring occasionally with a wooden spoon, until the mushrooms have released their liquid and are beginning to brown, about 15 minutes. Add the onion, carrot, celery, garlic, thyme and remaining 1 1/4 teaspoons salt and cook until fragrant, the vegetables are soft and the mushrooms have browned, about 5 minutes longer. Deglaze with the white wine. Simmer until the liquid is almost entirely evaporated, about 4 minutes. Stir in the reserved bacon.

Step 3

Preheat the oven to 400 degrees F. Butter a 9-by-13-inch baking dish.

Step 4

Ladle and spread 1 cup bechamel on the bottom of the baking dish. Place a layer of pasta over the bechamel, breaking the pasta to fit the pan if needed. Spread 1 1/2 cups bechamel over the pasta. Spoon half of the mushroom filling over the bechamel and top with 3/4 cup Parmesan. Place another layer of pasta on top of the filling and repeat with the bechamel, the remaining mushroom filling and another 3/4 cup Parmesan. Layer on another layer of pasta and top with the remaining bechamel and Parmesan.

Step 5

Bake until golden brown and bubbly around the edges, 30 to 35 minutes. Allow the lasagna to rest for 15 minutes before serving.